ORA-01940: cannot drop a user that is currently connected

Cause : One is attempt to drop a user and user is currently connect to database, 

Resolution : we need to drop existing session of user, using below command :

ALTER SYSTEM KILL SESSION '*SID*, *SERIAL*';

See Full Post here, to find SID and SERIAL number of connected user (CLICK HERE)

Now you will be able to drop user using command 

Drop user username cascade;